This invention relates to a method for detecting the fully-charged state of a rechargeable battery (merely referred to as "battery" hereinafter just for simplification) such as a Ni-Cd battery, a Ni-MH battery or the like, when charging it by means of a battery charger.
When charging a battery such as a Ni-Cd battery, a Ni-MH battery or the like, by means of a battery charger, it is typically observed that the terminal voltage VB of the battery drops when the battery is fully charged. Thus, by using this characteristic, the fully-charged state of a battery has been detected by measuring the voltage drop (referred to as -.DELTA.V hereinafter) in the terminal voltage VB of the battery during the charging process.
FIG. 2 is a block diagram showing an example of the typical arrangement of a conventional battery charger which makes use of the above-mentioned battery characteristic.
In such a battery charger as shown in FIG. 2, the terminal voltage VB of the battery BAT is attenuated by an attenuator (ATT) 1 and is inputted to an analog/digital converter 2 (referred to as ADC hereinafter). The output signal S2 from ADC 2 is inputted to and is read by a central processing unit 3 (referred to as CPU hereinafter). When the terminal voltage VB drops and the output signal S2 is reduced, it is judged based on a program stored in advance in a Read Only Memory 4 (referred to as ROM hereinafter) that charging of the battery BAT has been completed, and the supply of output current S5 from a constant current circuit 5 to the battery BAT is terminated.
In such a conventional battery charger as described in the above, in order to detect the amount of voltage drop -.DELTA.V in the terminal voltage VB, the amount -.DELTA.V has undergone an analog/digital conversion (referred to as A/D conversion hereinafter) by means of the ADC 2, of which the quantization bit number is 8 or 10 bits, for instance. In case a small charging current is used for charging the battery, however, the change in the voltage drop amount -.DELTA.V becomes so small that a more accurate ADC, for instance one having a quantization bit number more than 10 bits, is required for detection. This should necessarily lead to a high cost for battery charger.
It is generally known that the peak value of, and the amount of the voltage drop -.DELTA.V in the terminal voltage VB change depending not only on the kind of battery but also on the ambient temperature of the battery. Despite the existence of such knowledge, the detection of the quantity -.DELTA.V has been executed thus far without paying any attention to changes caused by surrounding conditions. Especially, when the ambient temperature is high, the voltage drop quantity -.DELTA.V becomes so small that it is erroneously detected, whereby accurate detection of a folly-charged state of a battery has been impossible. In the case of a lithium ion battery, voltage drop in the terminal voltage VB does not occur even when the fully-charged state of the battery is established, so that it is not possible to detect the fully-charged state of such battery.
Further, in order to improve the accuracy of detection of the fully-charged state of a battery, it might be tried in some cases to amplify the terminal voltage VB. However, if the terminal voltage VB is simply amplified, and the amplified terminal voltage exceeds the allowable input voltage of the ADC 2, it would become impossible to secure a dynamic range.